Magnesium bell housing.
This is a great chunk of magnesium. From the breathless eBay listing:

UP FOR AUCTION WE HAVE A TILTON MAGNESIUM BELL HOUSING W/ TRANSMISSION MOUNT / ADAPTER - RAN CHEVY - HOUSING DOES HAVE MINOR CRACK. SET COMES FROM MAJOR NASCAR TEAM AND IS RACE PROVEN- AND READY TO GO!!

I wasn't sure this was real magnesium, but since it was cracked already, I broke off a small piece and tried to light it. I was just about to give up and conclude that it was actually aluminum, but I know how hard magnesium can be to light sometimes. Sure enough, after about 30 seconds with a propane torch, the little bit caught and burned very brightly for longer than you might expect. No question about it, this is solid magnesium (though probably with a few percent of alloying elements).Source:eBay seller raceprovenpartsContributor:Theodore GrayAcquired:2 April, 2009Text Updated:8 April, 2009Price: $75Size: 14"Purity: >90%